The nervous system of vertebrates is divided into the central nervous system, comprised of the brain and spinal cord, and the peripheral nervous system, consisting of the outlying nerves (22). The axons of most nerve cells are covered with a myelin sheath, a stack of specialized plasma membranes. Glial cells that wrap around the axons produce the myelin sheath. In the CNS, these glial cells are called oligodendrocytes. Each region of myelin formed by an individual oligodendrocyte is separated from the next region by an unmyelinated area called the node of Ranvier (22).
The myelin sheath acts as an electric insulator of the axon and all electric activity in axons is confined to the nodes of Ranvier.
One of the more common types of CNS diseases among human adults is multiple sclerosis. This condition is a demyelinating disease. Multiple sclerosis is a chronic, frequently progressive, inflammatory CNS disease characterized pathologically by primary demyelination and axonal injury. In multiple sclerosis patients, conduction of action potentials by the demyelinated neurons is slowed (22). Even though the etiology of multiple sclerosis is unknown, several immunological features of multiple sclerosis, and its moderate association with certain major histocompatibility complex alleles, has prompted the speculation that multiple sclerosis is an immune-mediated disease (17, 33, 55). An autoimmune hypothesis is supported by the experimental autoimmune (allergic) encephalomyelitis (EAE) model, where the injection of certain myelin components into genetically susceptible animals leads to T lymphocyte-mediated CNS demyelination (58).
Some researchers view activated T lymphocytes as a trigger of multiple sclerosis. They postulate that once T lymphocytes traverse the blood brain barrier (BBB) into the CNS parenchyma, they are reactivated following antigen presentation by microglia (53). The entry of leukocytes into tissues is a multi-step process that includes adhesion onto endothelial cells and transmigration across the endothelial barrier.
Recent evidence suggests that the expression of matrix metalloproteinases (MMPs) by leukocytes is required for T lymphocytes to enter the CNS parenchyma (42, 77, 88). An MMP is a proteolytic enzyme that possesses an active site where an invariant zinc binds to cysteine residues in the propeptide region of the MMP, keeping the MMP in an inactive state (50, 89). Activating agents disrupt the cysteine-zinc interaction to expose the active site so that catalysis can occur.
The inappropriate expression of MMPs is speculated to be involved in the pathogenesis of malignant gliomas, stroke and Alzheimer's disease. Furthermore, several lines of evidence suggest a role for MMPs in the disease process in multiple sclerosis (37, 91). In this regard, immunohistochemically identified MMPs (specifically MMP-3, -7, -9 and -12), on microglia, astrocytes and infiltrating leukocytes, have now been documented by several groups to be present in the autopsied brains of multiple sclerosis subjects (4, 18, 21, 47, 57) and in the brains of EAE animals (16, 37). Analyses of serum samples reveal levels of MMP-9 to be significantly increased in multiple sclerosis patients compared to healthy controls; within the multiple sclerosis population, serum MMP-9 levels are higher during clinical relapse relative to periods of stability (40). In addition, serum MMP-9 levels are correlated with the number of gadolinium-enhancing lesions detected by magnetic resonance imaging (MRI) (40, 84).
In correspondence with a pathogenic role of MMPs, several hydroxamate-based agents developed to inhibit the activity of MMPs (e.g. GM6001, Ro31-9790 and BB-1101) were found to alleviate the incidence and severity of EAE (32, 34, 43, 52).
A prominent method of microglia activation is believed to be a non-antigen-specific interaction between these cells and T lymphocytes, which generates cytokines within the CNS milieu (19, 20, 67). This contention is supported by several findings obtained in culture studies. For instance, human T lymphocytes and human microglia interact to generate significant amounts of TNF-α and IL-10 (14, 15). The effect of T lymphocytes on microglia is equipotent to that of lipopolysaccharide, a very effective microglia stimulator. Researchers found additional support in the observation that the interaction between T lymphocytes and microglia does not require antigen or MHC restriction. Experiments have also shown that in a facial nerve resection model in mouse, T cells infiltrated the CNS and aggregated around microglia, and that this was correspondent with an increase in IL-1β and TNF-α (61). In a graft-versus-host disease (GVHD) model, activated microglial cell clusters were invariably intimately associated with T cell infiltrates (68).
TNF-α can influence lymphocyte trafficking across endothelium by up regulating the expression of various adhesion molecules involved in this process (76), and is implicated in the process of demyelination. Indeed, TNF-α directly induces in vitro the apoptotic death of the myelin-producing cells in the brain, the oligodendrocytes (26, 45, 71), and intravitreal injection of TNF-α causes demyelination of mouse optic nerve axons (12). In addition, TNF-α is pro-inflammatory. The level of TNF-α is found to be elevated in the serum, cerebrospinal fluid, and brain lesions of multiple sclerosis patients, and is correlated with the disease activity (13, 35, 62, 72). TNF-α is also implicated in the pathogenicity of EAE—the administration of antibodies to TNF-α or soluble TNF-α receptors prevents the transfer of EAE and abrogates autoimmune demyelination (63, 69, 70).
IL-12 is another pro-inflammatory cytokine which has a key role in switching uncommitted T lymphocytes to the pro-inflammatory Th1 subset which secretes IFN-γ and TNF-α/β (91). IL-1β also promotes an inflammatory response and has been associated with multiple sclerosis (66).
The Th2 subset produces IL-4, IL-10 and IL-13, regulates humoral immunity and decreases local inflammation. Both IL-4 and IL-10 can inhibit the differentiation of naive precursors into Th1 cells (91). IL-4 promotes the activation of B lymphocytes and macrophages and also stimulates class switching of antibodies (22).
IL-13 is a Th2 cytokine with important immunomodulating activities. The best known IL-13 mediated function is its ability to drive the differentiation of naive CD4+ T cells towards a Th2 phenotype (75). The anti-inflammatory functions of IL-13 include the suppressive effect on the production of pro-inflammatory cytokines by activated monocytes or by alveolar macrophages (81), the induction of 15-lipoxygenase (51) and the inhibition of prostaglandin E2 (PGE2) formation (27). Functions of IL-13 in the CNS and on glial cell functions are not well defined (87).
IL-10 is an anti-inflammatory cytokine produced by a variety of cells, including monocytes/macrophages, T lymphocytes, and mast cells. In the CNS, potential sources of IL-10 include microglia (86) and astrocytes (49). IL-10 has important anti-inflammatory properties. First, IL-10 inhibits the production of proinflammatory cytokines by many cell types, including those of the mononuclear phagocytic lineage; indeed, IL-10 was shown to inhibit the production of TNF-α and IL-12 produced by monocytes, macrophages, and microglia (3, 9, 10, 24, 38). Also, IL-10 plays a role in causing T lymphocytes to undergo anergy (inactivation or unresponsiveness) (2). Other anti-inflammatory functions of IL-10 include its inhibitory effect on the process of antigen presentation. Treatment of macrophages/microglia with IL-10 down-regulated the expression of molecules essential for presentation of antigens, such as MHC class II (24) and the costimulatory molecules B7-1 and B7-2 (36). Finally, the role of IL-10 as an anti-inflammatory molecule is supported by the phenotype of IL-10-deficient mice; these mice develop chronic colitis, which appears to be mediated by the proinflammatory Th1 cells (8, 23, 39).
The afore-mentioned cytokines are inducible, meaning that they are produced in response to certain stimuli. In contrast, IL-6, which activates B-lymphocytes (22), is constitutive.